An oxide ion conductive film used as a solid electrolyte film for fuel cell is required to allow transport of an oxide ion but does not allow electron conduction and gas permeation. Therefore, the oxide ion conductive film tends to become thicker. On the other hand, a thin oxide ion conductive film is preferable in the view of facilitating ion transportability.
A known conventional oxide ion conductive film with the highest conductivity is a sintered polycrystalline ceramic (Gd0.1Ce0.90O2-δ, La0.8Sr0.2Ga0.8Mg0.2O3-δ) (non-patent documents 1 through 3). In the oxide ion conductive films in these documents, it is necessary to be operable at 500° C. or higher even though the oxide ion conductive films are such a thin film of, e.g. 10 μm in thickness, otherwise the oxide ion conductive films cannot attain Ras<0.2 (Ωcm2), which should be satisfied for an oxide ion conductive film to be practically used in fuel cells. Further, to reduce the operation temperature, it is necessary to make the oxide ion conductive film thinner. A sintered polycrystalline ceramic having a thickness of 10 μm or less causes problems in workability and mechanical strength because gas molecules are easy to disperse at particle boundary. As a result, the sintered polycrystalline ceramic does not work as an oxide ion conductive film in the end.
Namely, no oxide ion conductive film exists which works as a conductive film for fuel cell at less than 500° C.
Moreover, high temperature, 1000° C. or more, is required to manufacture a conventional oxide ion conductive film which is made of ceramic. This causes a problem of performance degradation by a reaction with other constituent materials for fuel cell.
On the other hand, a proton conductive film is also used as a solid electrolyte film for fuel cell. However, most of them are organic proton conductive films typified by Nafion (registered trademark) and one made of inorganic materials is only reported as a kind of ceramic (non-patent document 4). An organic proton conductivity film, in particular, is not adequate to work at high temperature. In consideration of catalyst cost and utilization in cars and the like, it is preferable to work at higher temperature (non-patent documents 5 and 6).
It is expected to expand the ranges of workable temperature and adaptability by obtaining a conductive film which has both characteristics of oxide ion conductivity and proton conductivity. In addition, it is expected to improve the efficiency of a catalyst when the film is used in fuel cell.
Non-patent document 1: Nature 414: 345-352 (2001)
Non-patent document 2: Electrochem. Solid State Lett. 8: A389-391 (2005)
Non-patent document 3: Jpn. J. Appl. Phys. 43: 299-302 (2004)
Non-patent document 4: Electrochem. 68: 154-161 (2000)
Non-patent document 5: J. Power Sources 108: 139-152 (2002)
Non-patent document 6: J. Power Sources 152: 200-203 (2005)